1. Field of the Invention
The present invention relates to an image processing device such as a digital television (DTV), a multi-function monitor, or a display system on chip (SoC), and more particularly, to an apparatus and method of filtering a digital image signal.
2. Description of the Related Art
In general, a transmitter transmits a digital image signal which necessarily includes a noise component due to the characteristics of a transmission channel or a display device. A receiver may improve such a noise component using a signal processing technique.
Most of the conventional methods of filtering a digital image signal to reduce noise using a signal processing technique adopt 2-dimensional spatial filtering or 1-dimensional temporal filtering. In addition, there have recently been suggested techniques for using motion compensation to exactly extract inter-field correlation information from a moving image sequence. However, in such a conventional digital image signal filtering method, a noise component deteriorates the accuracy of motion estimation. Also, such motion estimation includes complicated operations and increases the complexity of hardware.
Moreover, an image, which has passed through a noise reduction filter, is generally less sharp than the input image. This results from the characteristics of a low-pass filter (LPF) of the general noise reduction filter. Thus, most conventional digital image signal filtering methods perform noise reduction filtering and then adopt a sharpness enhancement filter, which enhances a high pass component, so as to prevent the deterioration of the sharpness of an image.
Conventional methods using spatial filtering are disclosed in a paper entitled “Noise Reduction for MPEG type of Code” by L. Yan, 1994 and published in IEEE International Conference Acoustic, Speech and Signal Processing, a paper entitled “I.McIC:a Single-chip MPEG-2 Video Encoder for Storage” by A. van der Werf, November, 1997 and published in IEEE Journal of Solid-State Circuits, vol. 32, No. 11, and European Patent No. EP0878776 by Mancuso et al. published on Aug. 27, 2003. The conventional spatial filtering method disclosed in European Patent No. EP0878776 determines whether a pixel to be filtered is a smooth pixel using a fuzzy logic process so as to adaptively employ low-pass filtering. However, such a conventional spatial filtering method deteriorates filtering performance when a noise level is high.
Also, a conventional method using motion compensation is disclosed in a paper entitled “Noise Reduction Filters for Dynamic Image Sequences: a Review” by Katsaggelos, September 1996 and published in Processings of IEEE, vol. 83. The disclosed method requires a large amount of operations to be executed due to excessively repeated operations and thus is costly and has difficulty in real-time realization.
A conventional method using temporal filtering is disclosed in a paper entitled “Noise Reduction in Image Sequences Using Motion Compensated Temporal Filtering” by E. Dubois et al., July 1984 and published in IEEE Trans. On Communications, vol. COM-32, p 826-831. In the disclosed method, an image is roughly divided based on a distance between a central pixel and a neighboring pixel in order to prevent pixels included in different regions in a window from being filtered. Only a minimal amount of blurring occurs at the edge of an image, in a method using temporal filtering, compared to a spatial filtering method. However, the deterioration of image quality by artifacts such as ghost tail occurs with an increase in the number of frames used for filtering.
A conventional temporal filtering method, which is an improvement of Dubois technique, is disclosed in a paper entitled “A Method of Noise Reduction on Image Processing” by S. Inamori et al., November, 1993 and published in IEEE Trans. on Consumer Electorinics, vol. 39, No. 4. In the disclosed method, a filtering function is turned on or off through motion detection and edge detection to prevent a ghost tail from occurring. However, the ghost tail still occurs and particularly, the filtering function is turned off even when a noise peak appears.
Furthermore, the previously-described conventional methods mostly reduce noise using a low-pass filtering technique. As a result, the sharpness of a filtered result deteriorates.